1. Field of the Invention
Embodiments of the present invention relate to the field of computer graphics and, in particular, to augmenting hand-drawn animation with three-dimensional (3D) secondary motion.
2. Description of the Related Art
Secondary motion is the motion of scene elements in response to the movement of the primary character. Secondary motion is often used to amplify the character's motion and personality via effects that appear to be driven by his motion. Examples of secondary motion include the swishing of a robe, or the sloshing of water in a pail. The animation of these effects is done after the character motion is finalized, either as a separate layer in hand-animation, or as a simulation in 3D animation. These elements do not, in general, have a distinct character or personality of their own, i.e., they are passive, not animate.
Passive effects such as cloth, fluids or particles have many degrees of freedom, are involved in frequent collisions, and have complex interactions with characters. These properties make cloth, fluids, and particles difficult to animate by hand either on paper or using computer software. As a result, a great deal of research has focused on the simulation of these effects with impressive results in research prototypes that have subsequently been incorporated into commercial animation software. 3D animation may take advantage of simulated effects when a character's location and pose in 3D space is known and can be used as a driving signal to simulate the cloth, fluids, or particles. Hand, or two-dimensional (2D), animation cannot directly benefit from the techniques developed for 3D animation as the motion of the character is only determined on the image plane, not in the full three dimensions required to drive and interact with a physical simulation.
There has been work in the past on mixing 2D and 3D elements in a scene. One of the most well remembered works is Tarzan's Deep Canvas, where a 3D background is mixed with a 2D hand animated foreground character. However, the techniques used to produce works such as Tarzan's Deep Canvas did not enable the kind of physical interaction between the hand-drawn elements and the 3D elements needed to create secondary motion; for example, ripples when Tarzan steps into a pool of water. In other words, the 3D scene elements did not physically interact with the 2D scene elements.